In practice, power boilers may include superheaters (SH), (i.e., heat exchangers, in which the temperature of high pressure steam produced in an evaporator is raised above a saturation temperature). Superheated steam is then conducted to a high pressure steam turbine to produce steam power. Many boilers also comprise a reheater, in which the temperature of lower pressure steam released from the high pressure steam turbine is raised again, in order to produce more power by an intermediate pressure steam turbine. In the following, the term “superheater” may refer to either an actual superheater or a reheater. The term supercritical (SC) boiler refers to a boiler having a steam temperature of at least about 550° C., whereas, for ultra supercritical (USC) boilers, the steam temperature is at least about 600° C. The use of increased superheat and reheat temperatures improves the cycle, and thus, the plant efficiency.
Usually, a power boiler comprises a superheater system consisting of multiple in-series-connected superheater sections, which are located in different parts of the boiler. Superheaters are generally called convective superheaters (CSH), into which heat is mainly conducted by hot flue gas, or radiant superheaters (RSH), which dominantly absorb heat by radiation. Radiant superheaters are arranged at the top of the furnace of a boiler to be in direct visibility to the flames in the furnace. For SC and USC pulverized coal firing boilers, the duty of the RSH is substantially greater than that for a supercritical boiler. Thus, a series RSH arrangement is often used to obtain the required steam enthalpy. The metal tube temperature of an RSH depends on the local heat flux and on the temperature of the steam flowing in the tube. The metal temperature can be especially high at the bottom of a radiant SH, facing the flame zone.
Superheating of saturated steam is usually started in a CHS arranged in the flue gas channel downstream of the furnace. From the CHS, the steam usually goes to an RSH arranged at the upper portions of the furnace. The RSH may comprise pendant tube coils or hanging panels of tubes, or divisional tubewalls arranged parallel to the flue gas flow. Steam leaving the RSH usually goes to an attemperator, where water is sprayed onto the steam, to bring down the steam temperature to its desired value. From the attemperator, steam finally goes to a pendant superheater (PSH) arranged behind the nose of the furnace or in a horizontal pass immediately downstream of the furnace for further superheating the steam before it leaves to a high pressure (HP) turbine. Steam exiting the HP turbine may be conducted back to the furnace for being re-superheated to the desired temperature in a reheater (RH). Steam, after being reheated, flows to the intermediate pressure (IP) turbine for further expansion. The RH is usually arranged in the horizontal pass downstream of the PSH, but it may, as well as the PSH for final superheating, in some cases, also be arranged as a radiant superheater at the top portion of the furnace.
Due to high flame temperature in the furnace, the durability of superheaters may suffer from overheating. German Patent No. 1012614 discloses an arrangement in which the tubes of a superheater are protected from overheating by special shield tubes leading steam to a convective superheater. Great Britain Patent No. 855,114 discloses a boiler having superheater tubes, closest to the flame in the furnace, protected from radiation by reheater tubes surrounding the superheater tubes. It is also known from U.S. Pat. No. 3,101,698 to make a platen superheater behind a furnace nose, in which third and fourth passes are arranged partially in parallel flow (i.e., so that horizontal radiation is directed to tubes of a third pass, which are in flue gas flow upstream of the fourth pass, to prevent overheating of the hotter outlet tube sections).
The above-mentioned prior art solutions may adversely alter the heat duty among the superheating stages, and thus, lower the thermal efficiency of the boiler, or they address primarily convective dominant heat transfer. Therefore, there still exists a need for an improved superheater.